ALSA: at73c213: manage SSC clock

Since commit 49af54ff ("misc: atmel-ssc: prepare clock only when
request") the SSC driver expects clients to manage the SSC clock.
This updates the at73c213 driver to enable the SSC clock whenever it is
needed.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>